Border Gateway Protocol (BGP) is a protocol for exchanging routing and reachability information between autonomous systems (ASes). An AS is a set of routers under a single technical administration. An AS typically employs an interior gateway protocol (IGP) to exchange network topology information among routers within the AS. Examples of IGPs include link-state routing protocols such as Intermediate System to Intermediate System (IS-IS) and Open Shortest Path First (OSPF).
Border Gateway Protocol Link-State (BGP-LS) uses BGP as a carrier for network topology and reachability information collected by an IGP. BGP-LS allows a BGP speaker to share network topology and reachability information collected by the BGP speaker (e.g., link-state information collected using IS-IS or OSPF) with a peer BGP speaker (e.g., another BGP speaker located in another AS) via BGP. For this purpose, BGP-LS defines a link-state network layer reachability information (NLRI) encoding format that is used to provide network topology and reachability information to external components. Each link-state NLRI may describe either a node, a link, or a prefix.
Support for BGP-LS adds non-trivial overhead to IGP operation in terms of processing and identifying changes in the link-state database. This is made worse in situations where the IGP continuously receives updated link-state information, for example, due to frequently changing traffic engineering (TE) data. The continuous updates may cause the IGP to relay excessive amounts of link-state information to the peer BGP speaker, which can lead to further churn and potentially impact the entire network.